Silicon carbide (SiC) is a material with excellent mechanical properties at high temperatures. SiC fibers are of great interest for the fabrication of composite materials, especially composite materials for use in high temperature structural applications. This invention concerns a method for producing silicon carbide (SiC)-based fibers with improved creep resistance and high tensile strength, and the fibers so produced, by exposing the sintered fibers to nitrogen gas at temperatures above the sintering temperature and exposing the fibers to carbon monoxide gas at lower temperature, with the exposure steps occurring in either order.
There are many methods of forming SiC ceramics. SiC ceramics are often prepared by forming and consolidating fine SiC particles into a desired shape and subsequently heat treating (i.e., sintering) the "green" shape in order to eliminate the interparticle pores (i.e., void space) and to obtain a high-strength body with high relative density (i.e., with little or no residual porosity).
SiC ceramics are also prepared by other methods, especially by chemical vapor deposition (CVD) and by heat treatment of organosilicon polymers. For example, organosilicon polymers have been used to prepare fine SiC particles, fibers, bulk samples, coatings, etc. Continuous SiC fibers with fine diameters (e.g., .about.5-25 .mu.m) are generally prepared from organosilicon polymers. Some SiC fibers (and other types of samples) prepared using organosilicon polymers develop fine SiC crystallites and fine pores at some stage during processing and, hence, a sintering step is required to ultimately produce a dense (low porosity), high-strength SiC sample.
The typical sintering temperatures for preparing dense SiC are in the range of approximately 1700-2300.degree. C. The required temperature is highly dependent upon the size of the SiC particles (or crystallites) which comprise the porous body that is being sintered. For example, SiC bodies fabricated from the more conventional powder processing routes generally require higher sintering temperatures (.about.1900-2300.degree. C.) and this results in sintered bodies comprised of coarser grain sizes (&gt;1 .mu.m). In contrast, organosilicon polymer-derived SiC bodies (e.g., fibers) can be sintered at lower temperatures (e.g., .about.1700-1900.degree. C.) and, consequently, the resulting grain sizes are usually smaller (&lt;1 .mu.m).
For high temperature applications, it would be desirable to have SiC fibers with high strength and high resistance to creep. As discussed by DiCarlo (in Composites Science and Technology, 51 213-222, 1994), it is difficult to achieve both of these properties simultaneously in SiC fibers. The strength of SiC fibers is controlled by "flaws" (e.g., voids, particulate impurities, kinks, grains, etc.). When other flaws are avoided through careful processing, the strength is controlled by the size of the SiC grains comprising the fibers. Hence, it is generally observed that carefully processed organosilicon polymer-derived SiC fibers have much better strength compared to powder-derived SiC fibers because the grain sizes are much smaller. The tensile strengths for organosilicon polymer-derived SiC fibers are typically in the range of .about.2.0-3.5 GPa. (Fiber tensile strengths .gtoreq.2 GPa are desirable for the development of high-strength, fiber-reinforced composites.) In contrast, the tensile strengths for powder-derived SiC fibers are usually much lower, e.g., .about.0.5-1.5 GPa.
The creep resistance of SiC fibers is usually highly dependent upon grain size also. However, the effect of grain size on creep behavior is the opposite of its effect on strength. The creep resistance increases as the grain sizes increases. Hence, powder-derived SiC fibers usually show better creep resistance than polymer-derived SiC fibers. Thus, it has not been possible in the past to make SiC fibers with both high tensile strength and high creep resistance. The difficulty in simultaneously achieving high tensile strength and high creep resistance in SiC fibers was demonstrated by Takeda et al. (in Ceram. Eng. Sci. Proc., 17[4] 35-42, 1996) using the organosilicon polymer-derived fibers known as HI-NICALON TYPE S.TM. which are produced by Nippon Carbon Co. As-prepared HI-NICALON TYPE S.TM. fibers have an overall composition close to that of stoichiometric SiC, low-oxygen-content (.ltoreq.0.2 wt %), high tensile strength (.about.3 GPa), and fine diameter (.about.12 .mu.m). Takeda et al. heat treated these fibers at temperatures up to 1900.degree. C. for 10 hours in an argon atmosphere. The fiber tensile strengths decreased with increasing heat treatment temperature, reaching values below 1 GPa for the longest heat treatment carried out at the highest temperature. X-ray diffraction line broadening measurements showed that the SiC crystallite (grain) sizes increased as the re-heat treatment temperature (and/or time) increased. Hence, the decrease in tensile strength with heat treatment was attributed to the increase in grain sizes. The creep behavior of the SiC fibers was assessed using a bend stress relaxation (BSR) test. In contrast to the strength behavior, the creep resistance of the fibers increased as the grain size increased. Fibers given the longest heat treatment carried out at the highest temperature showed the best creep resistance, as assessed by the BSR test. Heat treatments of the organosilicon polymer-derived HI-NICALON TYPE S.TM. fibers at the highest temperature and longest time resulted in fibers that were similar to powder-derived fibers in terms of good creep resistance and low strength values. This combination was observed because the heat treatments resulted in fibers with relatively large grain sizes.
It is well known that SiC ceramics with high relative density (i.e., low residual porosity) and fine grain sizes are desirable in order to attain high strength. However, it is very difficult to prepare pure SiC with high relative density and fine grain sizes by sintering methods, especially by pressureless sintering methods. In samples comprised of fine particles (or fine crystallites), pure SiC generally undergoes coarsening (growth) of particles (crystallites) and pores during high temperature heat treatment because of the dominance of surface diffusion and/or vapor phase diffusion processes. Thus, very little densification (i.e., pore removal) occurs in pure SiC during high temperature heat treatment. As a result of this problem, additives (i.e., "sintering aids") are used to enhance the densification (and prevent coarsening) during sintering and, thereby, allow the fabrication of SiC with high relative density and fine grain sizes. Several additives have been found effective as sintering aids for SiC, but boron-containing compounds are the most commonly used additives. Varying amounts of boron compounds have been reported as effective for sintering (e.g., 0.2-5 wt %), but boron concentrations on the order of .about.0.5-1 wt % are most common. When sintering aids are used, the typical sintering temperatures for preparing dense SiC are in the range of approximately 1700-2300.degree. C. As noted earlier, the required temperature is highly dependent upon the size of the SiC particles (or crystallites) which comprise the porous body that is being sintered.
In addition to controlling the sintering temperature and using the proper amount and type of sintering additive, it is also important to control the gas atmosphere during sintering of SiC in order to achieve high relative density and fine grain size. There have been numerous studies in which SiC was sintered in various atmospheres. It is well known that oxidizing atmospheres are undesirable, while atmospheres which are usually referred to as "inert" or "chemically inert" are considered desirable. Argon is the most common atmosphere used in sintering of SiC. Helium, nitrogen, and vacuum have also been widely reported as useful atmospheres for sintering of SiC.
As described in more detail below, there have been many prior studies in which SiC was fabricated using nitrogen as the atmosphere during the sintering (densification) process. In the prior art, however, heat treatments in nitrogen-containing atmospheres have only been carried out at the sintering temperatures or at temperatures below the sintering temperatures. Furthermore, the prior art specifically teaches against heat treating SiC at temperatures above that required for the purposes of sintering. According to the prior art, this will result in a loss of strength of the SiC body. In contrast, the current invention teaches the surprisingly result that high strengths can be maintained in samples which are heat treated in a nitrogen-containing atmosphere at temperatures higher than the sintering temperature.
Prochazka in U.S. Pat. Nos. 3,853,566, 3,960,577, 3,954,483, 3,968,194, and 4,004,934 reported that small amounts of boron (especially when combined with small amounts of carbon) could be used to enhance densification and produce SiC bulk samples with high densities and fine grain sizes. Boron was incorporated by mixing SiC powders with boron-containing powders such as elemental boron, boron carbide, and boron nitride. It was also reported that gaseous boron trichloride could be used to vapor-phase dope boron directly into SiC powders during their synthesis by CVD methods. The minimum amount of boron needed for good densification was approximately 0.3 wt %, although higher amounts were generally used. Prochazka and many others researchers concluded that it is desirable to use the lowest possible temperature and shortest possible time that are needed to accomplish densification (i.e., elimination of porosity) during sintering of SiC. This is not only because of the cost savings, but also because prolonged heat treatment results in grain growth in the sintered body. Grain growth is considered undesirable in many cases because it usually results in a reduction in the strength of the sintered body. For example, in U.S. Pat. No. 4,004,934, Prochazka states: "An extended dwell [time] at high temperature is harmful because it brings about coarsening of the microstructure and consequently degradation of the mechanical properties. Thus, the shortest hold [at the sintering temperature] is preferable." Because Prochazka used powder processing methods, sintering and hot pressing temperatures were generally in the range of .about.1900-2100.degree. C. (with the lower temperatures in this range being used for samples which were hot pressed). From U.S. Pat. Nos. 3,853,566, 3,960,577, 3,954,483, 3,968,194, and 4,004,934, it is reported that boron-doped SiC bodies can be sintered to high relative density using several different gas atmospheres, including argon, helium, and nitrogen. In the first claim of U.S. Pat. No. 4,004,934, it is stated that a dense boron-doped SiC ceramic can be made by "sintering of the green body in an inert atmosphere chemically-inert with respect to silicon carbide at atmospheric pressure or below atmospheric pressure at a temperature of about 1900-2100.degree. C. . . . ." A nitrogen atmosphere was used in most of the examples in U.S. Pat. No. 4,004,934, so this patent teaches that SiC is chemically-inert in nitrogen.
Many other studies have been reported in which SiC is sintered in a nitrogen-containing atmosphere. In each case, the prior art teaches that the nitrogen atmosphere should be used at the lowest temperature required for densification. For example, Urasato et al. in U.S. Pat. No. 5,011,639 sintered green bodies prepared with SiC powders which had been mixed with .about.0.2-5.0 wt % of boron powder or a boron compound powder (e.g., boron carbide, boron oxide, titanium diboride). In this case, nitrogen was not considered an inert gas. The patent states that green bodies were ". . . sintered in an atmosphere of an inert gas containing nitrogen in a minor concentration. The inert gas here implied is a rare gas such as helium, argon, and the like. The concentration of nitrogen in the atmosphere of mainly the inert gas should be in the range from 0.005 to 5% by volume or, preferably, from 0.01 to 2% by volume." It is stated that "The furnace for sintering should be filled with nitrogen-containing inert gas throughout the sintering procedure." In addition, it is stated that "The temperature of sintering should be in the range of 1800 to 2200.degree. C. or, preferably, from 1950 to 2150.degree. C. When the temperature for sintering is too low, the green body cannot be fully sintered so that the sintered body would have somewhat decreased density. When the temperature of sintering is too high, on the other hand, undue grain growth and thermal decomposition of silicon carbide may take place so that the sintered body has a decreased electric resistivity and mechanical strength."
Boron-doped SiC bodies with high relative density have also been formed using mixtures of SiC particles and borosiloxane polymers. This was demonstrated by Burns et al. in U.S. Pat. No. 5,112,779 using sintering temperatures in the range of 2000-2200.degree. C. to achieve densification. Nitrogen was considered an inert atmosphere suitable for sintering. It was stated that "Inert atmospheres are used for sintering. . . . For the purposes of this invention, an inert atmosphere is meant to include an inert gas, vacuum, or both. If an inert gas is used it may be, for example, argon, helium, or nitrogen."
Powder processing methods can be used to form SiC fibers, especially fibers with larger diameter (i.e., &gt;25 .mu.m). Frechette et al. in U.S. Pat. No. 4,908,340 and in Ceram. Eng. Sci. Proc. 12[7-8] 992-1006, 1991 (by F. Frechette, B. Dover, V. Venkateswaran, and J. Kim) prepared SiC fibers by either melt spinning or dry spinning of mixtures of SiC particles and organic polymers. Boron was incorporated in the fibers by adding 0.2-1.0 wt % boron carbide powders to the spinning mixtures. Some fibers were heat treated in crucibles which had been coated with slurries containing boron carbide in order to form boron-containing gases during the sintering operation. SiC fibers were sintered at temperatures in the range of 1900-2150.degree. C. in an argon atmosphere or at 2300.degree. C. in a nitrogen atmosphere in order to obtain substantially dense (i.e., low porosity) fibers. (It was reported in U.S. Pat. Nos. 4,004,934 and 4,908,340 and elsewhere that higher sintering temperatures were required for densification (i.e., pore removal) when nitrogen was used as the sintering atmosphere.)
The fibers produced by the method of Frechette at al. are usually referred to as Carborundum SiC fibers in various published scientific papers. As expected from the earlier discussion, these powder-derived fibers had relatively low tensile strength (.about.1 GPa) because the grain size were relatively coarse (i.e., average grain sizes were greater than 1 .mu.m and individual grains up to .about.10 .mu.m were observed). However, the fibers showed excellent strength retention after heat treatment at elevated temperature. In addition, creep resistance was excellent, as indicated from both BSR tests and from in-situ creep tests carried out at high temperatures.
Bolt et al. in U.S. Pat. No. 4,942,011 prepared SiC fibers by spinning mixtures of SiC particles and organosilicon polymer. One of the sintering aids used was boron carbide. The concentration of the sintering aids was 0.2-5% based on the SiC weight. Fibers were heat treated (sintered) at temperatures in the range of 1900-2000.degree. C. in an argon atmosphere, although it was indicated that inert or reducing atmospheres could be used.
Birchall and Clegg in U.S. Pat. No. 5,063,107 prepared SiC fibers in a similar manner to Frechette et al. and Bolt et al. They used mixtures of SiC particles and organic polymers. They also utilized known sintering aids, such as boron. Sintering was carried out at 2040.degree. C. for 20 min in an argon atmosphere. It was stated in the patent that "The later stage of the process of the invention will generally be effected in an inert atmosphere by which we mean in an atmosphere which is unreactive with the silicon carbide at the temperature of heating." It is also stated that "It may also be necessary to avoid the use of nitrogen as nitrogen may react with the silicon carbide to produce silicon nitride."
Organosilicon polymers are generally used to produce SiC fibers with fine diameter (i.e., &lt;25 .mu.m). Yajima et al. in U.S. Pat. Nos. 4,052,430, 4,100,233, 4,220,600, and 4,283,376 developed processes for preparing polycarbosilane polymers and SiC-based fibers from polycarbosilane polymers. The processes developed by Yajima et al. are the basis for NICALON.TM. fibers which are manufactured by Nippon Carbon Co., Ltd. A similar commercial process involves using a titanium metal-modified polycarbosilane (i.e., a polytitanocarbosilane) polymer and is the basis for TYRANNO.TM. fibers which are manufactured by Ube Industries Ltd. Yajima et al., in some of the earlier cited patents and in U.S. Pat. No. 4,152,509, also reported that SiC fibers could be prepared from polycarbosilane polymers which were synthesized from a polysilane and a phenyl-containing polyborosiloxane. The organosilicon polymer fibers produced by the various methods of Yajima et al. are pyrolytically decomposed to SiC-based fibers by heat treatment at temperatures usually .ltoreq.1300.degree. C. Pyrolysis was usually carried out in nitrogen or argon atmospheres or under vacuum. The fibers produced by these methods do not have good thermomechanical stability. The fibers degrade extensively and become extremely weak when heat treated at temperatures above approximately 1200-1400.degree. C. This is because of carbothermal reduction reactions that occur as a result of large amounts of oxygen and excess carbon in the SiC fibers.
Wallace et al. in U.S. Pat. No. 5,139,871 reported that the thermomechanical stability of organosilicon polymer-derived SiC fibers, such as TYRANNO.TM. fibers, can be improved by certain heat treatments in a nitrogen atmosphere. It was specified that heat treatments could be carried out between 800-1800.degree. C., with a preferred range of 1100-1600.degree. C. It was shown that heat treatments in a nitrogen atmosphere alone did not result in any improvement in the thermomechanical properties of the fibers. However, a small improvement in the thermomechanical properties of the fibers was observed when the fibers were placed in intimate contact with carbon particles during the heat treatment in nitrogen. Nevertheless, the fibers still had low strengths (i.e., &lt;1.5 GPa) after heat treatment at only 1600.degree. C.
Takamizawa et al. in U.S. Pat. No. 4,604,367 prepared boron-doped SiC-based fibers using organoborosilicon polymers which were prepared by reacting an organopolysilane with an organoborazine compound. The organoborosilicon polymers were melt spun into green fibers which were infusibilized by oxidative or irradiative cross-linking methods and subsequently pyrolyzed to form SiC-based fiber containing boron and nitrogen. Takamizawa et al. indicated that heat treatments could be carried out in vacuum or inert gas atmospheres at temperatures up to 1800.degree. C., although the preferred heat treatment temperatures were &lt;1600.degree. C. Nitrogen was considered a suitable gas for the pyrolysis process. The resulting fibers showed better strength retention upon high temperature heat treatments compared to NICALON.TM. fibers, but the fibers still started to show decreased strengths after heat treatments at temperatures above approximately 1100-1200.degree. C. and the strengths were too low for the fibers to be useful at temperatures above approximately 1500.degree. C.-1600.degree. C.
Researchers at Dow Corning have reported in U.S. Pat. Nos. 5,071,600, 5,162,269, 5,167,881, 5,268,336, 5,279,780 and 5,366,943 the fabrication of boron-doped, low-oxygen-content SiC fibers with carbon-rich or near-stoichiometric composition which were prepared from several different organosilicon polymers (i.e., polycarbosilane, methylpolydisilylazane, polyorganosiloxane). In U.S. Pat. Nos. 5,071,600 and 5,162,269, for example, fibers were formed by melt spinning of low-molecular-weight polycarbosilane. Fibers were oxidatively cured to prevent melting of the fibers during subsequent heat treatment. The primary modification of the process of Yajima et al. was to incorporate sufficient amounts of boron (&gt;0.2 wt %) in the fibers so that high relative density and fine grain sizes could be obtained after sintering. The patents describe mostly gas-phase doping methods, in which it was indicated that boron-containing compounds (e.g., diborane, boron trifluoride, boron tribromide, boron trichloride, tetraborane, pentaborane, borazine, trichloroborazine) could be infiltrated into the fibers at temperatures in which the compounds were in the form of gases. The boron-containing gas could be introduced at various stages in the fiber fabrication process, but it must be present prior to the onset of sintering in order to prevent grain coarsening and to allow densification of the porous SiC fibers to occur. In addition to gas phase doping, the Dow Corning patents also describe instances in which the organosilicon polymer can be synthesized such that it contains some boron.
The Dow Corning patents indicate that oxygen, nitrogen, and excess carbon are eliminated from fibers (as gaseous volatiles) during a high temperature pyrolysis stage which occurs in the range of .about.1200-1600.degree. C. The fibers develop fine pores during this stage. The porosity is removed by sintering at a temperature typically in the range of 1800-1850.degree. C. (As noted above, boron must be present to prevent grain coarsening and allow pore removal to occur.) The Dow Corning patents teach against the use of nitrogen atmospheres for the high temperature processing steps. In U.S. Pat. No. 5,167,881, it is stated that "At pyrolysis temperatures above about 1400.degree. C., nitrogen-containing atmospheres are not preferred as nitrogen is not inert to the fibers under those temperature conditions. At high temperatures, truly inert gaseous atmospheres are preferred such as argon and/or helium." A similar statement is made in U.S. Pat. No. 5,268,336. (It should be noted that the patents use the term "pyrolysis" to encompass not only the process in which volatiles are removed at lower temperatures, but also the higher temperature sintering process.) The fibers produced in the Dow Corning patents do not contain any nitrogen. The first claim in U.S. Pat. No. 5,162,269 states that the fibers are "nitrogen-free." In U.S. Pat. No. 5,268,336, it is stated "The duration of the pyrolysis treatment should be sufficient to eliminate substantially all oxygen and/or nitrogen from the fibers."
Fiber characterization data, such as reported by Dicarlo (in Composites Science and Technology 51 213-222, 1994), Takeda et al. (Ceram. Eng. Sci. Proc., 17[4] 35-42, 1996), and Lipowitz et al. (Ceram. Eng. Sci. Proc., 16[4] 55-62, 1995), indicated that SiC fibers produced according to the methods described in the Dow Corning patents have thermomechanical stability which is much better than NICALON.TM. and TYRANNO.TM. fibers and similar to HI-NICALON TYPE S.TM. fibers. The fibers retained 96% of their original tensile strength after heat treatment at 1550.degree. C. for 10 hours in an argon atmosphere. The fibers retained 74% of their original tensile strength after heat treatment at 1800.degree. C. for 12 hours in an argon atmosphere. The fibers produced according to the Dow Corning patents are known as SYLRAMIC.TM. fibers.
Sacks et al. in U.S. Pat. No. 5,792,416 reported the fabrication of boron-doped, low-oxygen-content SiC fibers with near-stoichiometric composition, high relative density, and high tensile strength which were prepared using high-molecular-weight polycarbosilane polymers. Fibers were sintered in an argon atmosphere. In most examples, the fibers were sintered in the range of 1750-1800.degree. C. for 1 hour. If fibers were sintered at higher temperatures, shorter sintering times were used to produced fibers with high relative density and high strength. For example, sintering times were 12 and 2 minutes when sintering temperatures were 1845 and 1890.degree. C., respectively. Sintered SiC fibers produced by this method had an average tensile strength of .about.2.85 GPa. The fibers typically retained more than 90% of the original tensile strength after being re-heated in an argon atmosphere at 1800.degree. C. for 4 hours. However, the tensile strength decreased to .about.75% of the original strength after heat treatment in argon at 1950.degree. C. for 1 hour. Hence, heat treatments in an argon atmosphere which were carried out at higher temperatures or longer times than needed for sintering resulted in decreased tensile strengths of the fibers.
Sacks, in U.S. Provisional Appl. Ser. No. 60/055,424 entitled "Silicon Carbide Fibers with Boron Nitride Coatings," reported that boron-doped SiC fibers with high relative density (i.e., little or no porosity) can be heat treated in a nitrogen-containing atmosphere, at a temperature which is higher than the temperature used to originally sinter the fibers, to develop a boron nitride (BN) layer at the SiC fiber surface. In contrast to all previous results with SiC, the heat treatment at temperatures above the sintering temperature did not result in a decrease in the strength. The BN-coated SiC fibers prepared in this manner still had high tensile strengths, i.e., the strengths were approximately the same as the original uncoated SiC fibers.